Before finally weld the amplifier connectors pad the box inside with foam.
Rock wool is the most efficient but VERY uncomfortable to work with and without safety mask glasses and clothes can be even dangerous. The best technique for the fine tuning is trail and eror.
My box was filled with foam to about 75% of volume in the upper room of the box.
After messing with the rock wool I couldnt satnd it and moved to the acousta stuff that was supplied with the subwoofer amp set.
Connect and insert your amplifier to the subwoofer.
If you chose to completely stick it to the box with silicon it will be almost irreversible.
I did that.

Here is a picture from the first trail error sessions.